The reflectance and transmittance of dielectric films at nonzero angle of incidence show strong polarization effects, and for many applications these effects are not wanted. Costich has recently published on the reduction of these effects in interference films. The extension of his theory to the case of a multilayer inside a glass cube is possible, but an efficient design does not result. In this paper another method using quarter-wave layers only is developed.
Alfred Thelen, "Nonpolarizing interference films inside a glass cube," Appl. Opt. 15, 2983-2985 (1976)